The present invention relates to implantable stimulators, and more particularly to an implantable multichannel stimulator fashioned from a plurality of microstimulators connected in parallel, each microstimulator comprising a tiny self-contained stimulating device that is individually controllable from an external (non-implanted) power source, with each microstimulator providing its own stimulation output signal through a respective output electrode. One aspect of the invention relates to using such an implantable multichannel stimulator as an implantable cochlear prosthesis for electrically stimulating the auditory nerve of a profoundly deaf person.
Neurological disorders are often caused by neural impulses failing to reach their natural destination in otherwise functional body systems. Local nerves and muscles may function, but for various reasons, such as injury, stroke, or other cause, the stimulating nerve signals do not reach their natural destination. For example, paraplegics and quadraplegics have intact nerves and muscles and only lack the brain-to-nerve link, which stimulates the muscles into action.
While an implantable device that provides the missing brain-to-nerve link is not currently available, there are numerous stimulation devices available that provide electrical stimulation to excite muscle, nerve or other cells. Such devices have ranged in size and complexity from large, bulky systems feeding electrical pulses by conductors extending through the skin, to implanted stimulators which are controlled through modulated radio frequency (rf) signals, as set forth, e.g., in U.S. Pat. No. 4,524,774 (Hildebrandt), where modulated rf signals in the range of 27.12 MHz and 40.7 Mhz are used to control the implanted stimulator device. Through the selective positioning and control of such devices, it is thus possible to fashion a sequence of electrical stimulation signals that can control muscle, nerve or other cells in a desired manner, e.g., to excite arm muscles to move the arm, leg muscles to move the leg, and the like.
One area where externally-generated electrical stimulation pulses can be used to significant advantage is in the cochlea. The cochlea is part of the inner ear and resembles a snail shell within the temporal bone of the skull. It is conical in form and completes about 23/4 turns. It is about 5 millimeters (mm) in height and 9 mm in breath at its base. The cochlea, in cross section, has three main channels, an upper scala, a middle scala and a lower scala. These three channels are referred to respectively as the vestibular canal, the cochlear canal and the tympanic canal.
A membrane known as the basilar membrane separates the middle scala from the lower scala. Fibers of the auditory nerve (often referred to in the literature as the cochlear nerve) originate in the nerve cell bodies located in the spiral ganglion, which is located in the temporal bone, extending along and just medial to the three channels. Normally, these nerve cells contact hair cells that are located on the basilar membrane. Each hair cell terminates in about 20 cilia, or hair-like processes. Another membrane, known as the tectorial membrane, covers the cilia and causes a shearing action on the cilia when it moves with respect to the basilar membrane. Incoming sound waves into the middle scala cause the cilia to move or shear relative to the hair cells. Such movement or shearing action causes the hair cells to induce electrical pulses in the auditory nerve neurons going to the brain. The brain interprets such electrical pulses in the auditory nerve neurons as sound.
Unfortunately, the hair cells of the profoundly deaf are unable to generate electrical signals. Thus, no matter how much an incoming sound wave may be amplified, e.g., through the use of a conventional hearing aid device, the profoundly deaf are still unable to hear. It is thus apparent that there is a need in the art for a device that generates the electrical signals that would otherwise be generated by the hair cells of the profoundly deaf, thereby allowing such persons to experience the sensation of hearing.
Devices are known in the art that provide the sensation of hearing for the profoundly deaf by electrically stimulating the auditory nerve cells within the cochlea. Representative descriptions of such systems are presented below. However, all such known auditory-nerve stimulation systems suffer from one or more drawbacks, typically being much too large, too complex, too expensive, and/or ineffective to be of beneficial use to most persons who are profoundly deaf. What is needed, therefore, is a simple, inexpensive, easy-to-implant auditory-nerve stimulation system that can help larger numbers of the profoundly deaf to experience the sensation of hearing.
By way of example, a method of inducing hearing is taught in U.S. Pat. No. 3,751,605 (Michelson), wherein there is described a surgical procedure for implanting two opposing electrodes (conductors) within the lower scala of the cochlea. Incoming sound (pressure) waves are received and converted to electrical signals and amplified using electrical circuitry external to the ear. The amplified signals are then modulated and coupled to a receiver circuit implanted inside the ear, where the received signal is demodulated and applied to the two opposing electrodes. Such method, while representing a significant advance in the art at the time it was made, does not provide selective stimulation of the auditory nerve neuronal endings. That is, the system shown in the '605 patent is effectively a single channel system wherein all of the neuronal endings in contact with the electrode pair are effectively stimulated with the same signal.
Further, by way of example, U.S. Pat. No. 4,063,048 (Kissiah), which has been reissued as Reissue 31,031, teaches the use of a series of external, filter networks and generation of pulse signals of the same frequency as the audio signals. Such pulse signals are then applied by a plurality of electrodes to the portion of the cochlear nerve which normally transmits like signals in the normal hearing process. Thus, multiple channels are provided so that different portions of the cochlear nerve may be stimulated as a function of the frequency of the incoming signals. However, only the electrodes are implanted, which electrodes are connected to the female portion of a through-the-skin pin connector for making external connections with the external processing circuitry. Such pin connector, at best, may be source of irritation to the patient, and at worst, could easily be the source of infection. What is thus needed, is a simple, inexpensive multichannel stimulating system that does not require through-the-skin connections.
An additional example of a cochlear stimulator device is shown in U.S. Pat. No. 4,400,590 (Michelson). In the '590 patent, the cochlea is stimulated using a plurality of implanted electrodes driven by electronic circuitry and connected to the electrodes by way of a through-the-skin connector. The stimulating locations within the cochlea are selected as a function of the frequency of the electrical analog of the audio signal. One embodiment sums all frequencies and energizes all electrodes with the sum. Magnitudes are adjusted according to the particular user's requirements. However, only the multichannel electrodes are implanted in the cochlea, with a through-the-skin connection being made between the electrodes and the remaining electrical circuitry. While the '590 patent does mention that inductive and/or rf coupling could be used as a possible alternative to a through-the-skin connector, it does not teach how such coupling could be made, particularly in view of the fact that multiple channels are involved and any implanted circuitry would have to be extremely small, light weight, and consume very little power. What is thus needed is an auditory nerve stimulating system that avoids the use of through-the-skin connectors, while still providing multichannel operation.
Yet another example of a cochlear stimulator device is described in U.S. patent application Ser. No. 08/023,584, filed Feb. 26, 1993, incorporated herein by reference. The '584 application is a continuation of application Ser. No. 07/752,069, filed Aug. 29, 1993; which is a continuation in part of application Ser. No. 07/411,563, filed Sep. 22, 1989; all of which are assigned to the same assignee as the present application. Such application(s) describe a cochlear implant system known as the CLARION cochlear implant system, available from MiniMed Technologies, of Sylmar Calif. Such system includes an implantable cochlear stimulator (ICS) that drives a 16-contact intracochlear electrode, a wearable processor (WP), and a clinician's programmer (CP). The CLARION stimulator, or ICS, while representing a significant advance in the art, is nonetheless a relatively expensive unit due to the manner in which it must be manufactured. Moreover, should there be a component failure, it is possible that the entire ICS could malfunction depending upon the location and function of the failed component. What is thus needed is an implantable stimulator that is less expensive to manufacture and less susceptible to malfunctioning in the event of individual component failure.
The present invention addresses the above and other needs by providing a simple, reliable, inexpensive, easy-to-implant multichannel stimulation system, which system is particularly suited for stimulating the auditory nerve within the cochlea.